This invention relates to antennas combined with transmitters and/or receivers and particularly, but not essentially, to small, low voltage, substantially single frequency, transmitters/receivers and antennas which may have to be used without a good ground plane.
In the design of transmitters which are used for the location or tracking of birds, animals, vehicles, people or other objects it is desirable to use as short and light an antenna as is possible. Normal ly a standard quarter wavelength of wire is regarded as the minimum length to give sufficient signal power. Unfortunately a quarter wavelength antenna, if not associated with an adequate ground plane, can have a loss of up to ten dB relative to one with a good ground plane. On an animal, bird or person a good ground plane is usually not possible.
In the sport of falconry, small, substantially single predetermined frequency transmitters are attached to hunting birds in order that they can be tracked and located if lost or out of sight. For this use it is particularly important that the transmitters are as small and light as possible but capable of a emitting as strong a signal as possible without a good ground plane.
The frequencies used for this sport are in the VHF band (&gt;30 MHz) and are designated in each country in which it is practised. In the UK the legal frequency granted by the government is approximately 173 to 174 MHz. In USA, Canada, and Europe the frequency al loted for Falconry is approximately 216 MHz. In Germany frequencies of 151 to 155 MHz and 403 to 405 MHz are also used. In the Middle East frequencies in the range 215-220 MHz are used. The transmitters normally have a power less than 50 milliwatts, and are powered by batteries 1.5 to 6 volts nominal, preferably 3 to 4.5 volts nominal.
The transmitters presently in use have antennas made of spring steel wire which is quarter wavelength in length, that is approximately 33 cms for 216 MHz and 43.2 cms for 173 MHz. These antennas are extremely long compared with the overall dimensions of a bird and can get entangled in bushes, unbalance the bird or even cause it to be electricuted on high powered cables. It is thus highly desirable that the length of antenna be shortened for this use.
While the invention is mainly applicable to combined transmitters and antennas, it is also applicable to receivers and antennas, since it is sometimes useful for people to be able to carry light weight, small receivers which can be located in a pocket or the like and not easily visible. A combined receiver and transmitter can be connected to a common antenna. The term transmitter/receiver is intended to cover either on its own or both combined.
Prior Art
As stated above a straight wire quarter wavelength antenna is normally used.
A quarter wavelength of wire coiled into a helix has been used in some applications to reduce antenna length but the signal power compared with a straight wire is very much less and makes it unsuitable for use with low voltage transmitters.
A straight wire antenna with a coiled wire portion or inductance half way along it is also known. Although this effectively reduces antenna length it does not maintain radiated signal power in the absence of an adequate ground plane.
British Patent Specification No. 2046529 discloses an antenna having a coiled portion in series with a straight wire portion. This specification states that the straight wire portion has a length less than one quarter wavelength with the balance made up electrically by the coiled portion. However, the object of introducing the coiled portion is to compensate for the capacitive effects of the surroundings, particularly metal structures in a vehicle but not of the transmitter itself, and to allow the inductance to be varied after installation to cope with different surroundings. This Patent is not concerned with reducing the overall length of the antenna. They have only shortened the straight portion in order to be able to include a variable portion. The combined length of the coiled portion and the straight portion must be greater than one quarter wavelength, since only the part of the straight portion extending beyond its connection 24 to the coiled portion would have any radiating effect. The antenna is intended to be connected to a transmitter via a transmission line.